Electric induction motors are normally designed to provide rated load at rated speed. Simple control systems use voltage/frequency (V/F) curves that generate constant torque up to the knee of the curve. However, most such V/F curves are each tuned to a specific load and constant flux, yet many electric motors experience varying loads. If the load is less than the rated torque, excess flux is delivered resulting in less efficiency. Furthermore, electric motor systems designed for maximum load draw more power and generate more heat which reduces the reliability of those systems.
Optimal flux regulation provides a method of controlling efficiency. Normally the best efficiency is achieved by balancing copper and core power losses. When both reach the same power levels, the electric motor is generally running at the most efficient operating point. However, this is not the case at low torque loads. One method of controlling efficiency monitors estimated real power and adjusts the flux until the real power is at the lowest point. A problem occurs if the operating range is small due to current and voltage limits. Also, the starting flux point must be within the operating range or the electric motor will not generate the necessary torque. The current and voltage limit is set by the control hardware. The current limit is based on the control's inverter capability, and exceeding the current limit may damage the inverter. The voltage limit is based on the direct current (DC) supply. Controls with a power factor control (PFC) will maintain a relatively consistent DC voltage, while controls without a PFC will rely on the alternating current (AC) voltage supplying the control and must cut off at a predetermined low DC voltage level. Many controls now use vector control that separates the phase current (I) into two vector current components: The Id current vector regulates the flux, while the Iq current regulates the torque.
This background discussion is intended to provide information related to the present invention which is not necessarily prior art.